In a car equipped with an automatic transmission having a lockup clutch, the present invention relates to a method and an apparatus for controlling the car which controls both the engine and the automatic transmission to improve the fuel economy of the car and provide of comfortable riding of the car.
An automatic transmission equipped with a torque converter has a slipping loss due to the torque converter and has, therefore, a disadvantage of higher fuel consumption as compared with a manual transmission equipped with a mechanical clutch. As a result, there has been an increasing tendency that an automatic transmission having a lockup clutch built into the torque converter is being used to have an improved fuel economy of the car.
A lockup clutch mechanically transmits an engine torque directly to the transmission with a clutch except for starting or slow speed running of a car, to thereby avoid a slipping loss of a fluid in the torque converter. Accordingly, when the lockup clutch is being engaged (locked up), a torque is transmitted from the engine to the car shaft without the fluid so that fuel economy of the car can be improved.
From the viewpoint of improving the fuel economy, it is more advantageous that the car speed, when the lockup clutch is engaged, is set as low as possible. However, if a car is abruptly accelerated from the state of a low engine speed in a lockup state, there occurs an extreme torque variation, which makes it difficult for the fluid to absorb the torque variation, and a vibration occurs in the drive train as a result. This vibration is transmitted to the car body to generate a car body vibration and a booming noise within the passenger room. Accordingly, although a lockup at the time of a slow speed operation is desirable from the viewpoint of the fuel economy, the car speed when the lockup operation is started is limited from the viewpoint of the restriction of a car body vibration (torque variation).
A lockup control of the torque converter is disclosed in the Japanese Patent Unexamined Publication No. JP-A-61-136057 dated Jun. 23, 1986, for example. According to the invention disclosed in this publication, a lockup is cancelled when a torque variation has occurred.
According to the conventional automatic transmission, there has been a wide operation area in which the torque converter is used in a slipping state, or a lockup cancelled state, which has resulted in a small effect of improvement in the fuel economy due to the lockup clutch.